The exhaustive set of value of a for which `a-cot^(-1)3x=2tan^(-1)3x+cos^1xsqrt(3)+sin^(-1)xsqrt(3)m a yh a v e` solution, is `[-pi/4,pi/4]` (b) `[pi/2,(3pi)/2]` `[(2pi)/3,(4pi)/3]` (d) `[-(3pi)/6,(7pi)/6]`